The fabrication of gas discharge lamps requires that precise quantities of high purity mercury and alkali metals (e.g., sodium) be introduced into the gas envelope of the lamp. Of particular interest in recent years are high pressure sodium lamps which require vaporizable fills of sodium and mercury. These lamps have assumed commercial importance because of their high efficiency, typically in the range of 100 to 120 lumens per watt. The light output of high pressure sodium lamps is characterized by strong continuum radiation and a line spectrum richer than that associated with conventional mercury vapor lamps. High pressure sodium vapor lamps have been found particularly useful and effective in anti-crime lighting systems deployed in many urban areas.
A high pressure sodium vapor discharge may be created within a discharge tube formed from a high temperature, alkali-vapor resisting translucent polycrystalline alumina envelope with generally oppositely disposed electrodes. The operating pressure may range from 100 to 200 torr. Sodium, among the alkali metals, provides a high pressure discharge of the highest luminous efficiency and has relatively good spectral distribution. Mercury may be added to the sodium in the discharge tube as a buffer gas. Commonly a noble gas at approximately 15 torr pressure is placed in the tube as a starting gas.
In the preparation of these lamps, molten sodium-mercury amalgam has been dispensed into the gas envelope of the lamp by means of a vacuum needle pick-up. This technique is ineffective and poorly adapted to use on high volume manufacturing lines for several reasons. First, the ambient surroundings, materials, and equipment associated with the dispensing operation must be maintained at elevated temperatures, typically from 66.degree. to 220.degree. C., in order that the amalgam may remain in a molten state. Also, since the molten amalgam is extremely susceptible to oxide formation and since sodium will react with water, the dispensing operation must be performed in a controlled, inert water-free atmosphere. Finally, dosing needles employed to dispense the molten amalgam are continually clogged by sodium oxide floats or by decomposition of the needle itself from reaction with the corrosive alloyed sodium. The dosing of improper quantities of mercury and vaporizable sodium is a principal cause of high lamp rejection rates (often about 50 percent or more) associated with this process. There is also a health hazard associated with the use of a hot amalgam if the system should break and get toxic mercury in the atmosphere. In addition, hot sodium can explode if there is sufficient moisture in the atmosphere.
Another disadvantageous dosing procedure practiced by other lamp assemblers entails dispensing a carefully measured quantity of liquid mercury into a gas envelope of a lamp, inserting an open ended tantalum tube containing a measured quantity of solid sodium metal into the gas envelope, sealing the gas envelope, and heating the tantalum tube with a high frequency generator to vaporize the sodium. The procedure has several obvious disadvantages. First the liquid mercury may be partially retained in dosing conduits, thereby varying the composition of the fill. Sodium, exposed on the ends of the tantalum tube, may oxidize, thereby also varying the composition of the fill. Any sodium which is oxidized does not form an amalgam with the mercury. The procedure is a time consuming, multi-stage operation requiring the performance of two measuring and two dispensing steps, the sealing of the gas envelope, and the application of high frequency energy to vaporize the sodium. Finally, the lamp fabricated by this procedure will contain an extraneous piece of tantalum tubing within its gas envelope.
A need remains in the art for a fast, relatively simple and accurate procedure of dosing sodium amalgams into gas lamp envelopes.
An advantageous process and apparatus for the manufacture of discrete particles of metal halide particles is disclosed in U.S. Pat. No. 3,676,534.